Method for etching

ABSTRACT

Methods for etching metal oxide films, especially tin oxide.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a method for etching metal oxide thinfilms, particularly tin oxide films.

[0003] 2. Discussion of the Background

[0004] Transparent Conductors are electrically conductive thin films,typically less than 1 micron thick, which transmit a substantialpercentage of energy in the visible and/or solar bands of theelectromagnetic spectrum. They are used in photovoltaic devices and mostvisual displays of both the emissive (light generating) and passive(light modifying) types. Transparent conductors consist mainly of twotypes:

[0005] 1. Very thin metallic conductors (100-200 Å thick) such as silverand gold, and

[0006] 2. Non stoichiometric transparent conductive metal oxides (TCOs)optionally doped for enhanced conductivity.

[0007] Examples of TCOs include: Zinc Oxide, Indium Tin Oxide (ITO) andTin Oxide (TO) which is SnO₂ optionally but preferably doped withfluorine. The current standard for many display applications is ITO,mainly because of its ease of etching at moderately elevatedtemperatures in strong acid or oxidizing solutions. TO is not readilychemically etched in the fine line patterns required for modern displayand photovoltaic applications. In fact, for photovoltaic applications,current practice employs laser removal of the preferred TO film. Thismethod is slow and expensive since the material is removed (vaporized)by serial progression of the laser head. Such methodology is totallyinapplicable to display devices which may have thousands of lines orregions where the TCOs need to be removed to provide electricalisolation between conductive regions of the display. So, for displayfabrication, the preferred process for configuring the required filmpatterns is to deposit the TCO on the entire surface of the substrate(usually glass), apply an etchant resist material (mask) to thoseportions of the TCO that it is desired to preserve, and remove or etchthe unwanted material from the surface of the substrate.

[0008] TO has a number of advantages over competing TCO's for manyapplications:

[0009] 1. Low cost. Indium is some 37 times more costly than Tin. ITO isapplied in a sputtering chamber under clean room conditions. TO isapplied at temperatures above 500° C. where few foreign particlessurvive, and may be applied on the float glass line as the glass isformed.

[0010] 2. TO is very durable and less subject to damage during displayfabrication. It may also be directly connected to most flat cableinterconnects without additional metal deposits.

[0011] 3. TO is electrically stable in the 500° C. range which isencountered in some important display fabrication environments.

[0012] 4. TO forms cohesive bonds with the glass and/or alkali ionbarrier layers, such as SiO₂ and Al₂O₃ deposited betwixt the substrateand the TO layers to prevent electrolytic decomposition of the TCO. Thisfeature eliminates any concerns about TCO adhesion to the substrate.

[0013] For these and other reasons a low cost, reliable,production-prone process for etching TO is strongly desired in the art.

[0014] The etching of TO films with metallic zinc powder andhydrochloric acid (HCl) has a long history; typically the film iscovered with the powdered metal, then immersed in a bath of acid. In animprovement to this procedure by Kato and Fukai, disclosed in JapanesePatent Publication 4-69234 (Nov. 5, 1992), incorporated herein byreference, ferric chloride (FeCl₃) is added to the acid bath. Otherinnovations encountered in a review of the prior art which are unrelatedto our invention include the following, each of which have one oranother defects which have prevented their generalized adoption.

[0015] 1. U.S. Pat. No. 4,040,892, Sargent and Ghezzo (Aug. 9, 1977).The phosphosilicate glass mask and hot concentrated HI called for is notpractical in a production environment.

[0016] 2. U.S. Pat. No. 3,205,155 (Sep. 7, 1965), Van Natter. Safety anddisposal problems associated with alkali metal in amalgam etching aswell as high cost is the problem here.

[0017] 3. U.S. Pat. No. 4,009,061 (Feb. 22, 1977). Simon. Chromium(Cr⁺⁺) does not reduce TO to Tin in any reasonable time frame.

[0018] 4. U.S. Pat. No. 4,750,980, Hynecek et. al. (Jun. 14, 1988); U.S.Pat. No. 4,544,444, Chang (Oct. 1, 1985) U.S. Pat. No. 5,094,978,Miyagaki et. al. (May 6, 1992). These patents use plasma etching whichis costly and too slow.

BRIEF DESCRIPTION OF THE FIGURES

[0019]FIG. 1 shows a cross-section of a fine line masked pattern priorto etching.

[0020]FIG. 2 is a cross-section of a fine line pattern showing a perfectetch.

[0021]FIG. 3 shows a fine line etch pattern with a theoretical minimumundercut.

[0022]FIG. 4 shows preferred concentrations for an M-A-X etch.

[0023]FIG. 4 shows the preferred point (point 9) representing bestchoice (as determined by undercut, line definition and completeness ofthe etch) for our particular ≅3000 Å thick samples, supplied by AFGIndustries Inc. as their product““Comfort E²””, the specifications ofthe which are incorporated herein by reference. In finding the bestpoint we were guided by the principle of approaching the lower rightcorner of our designated range (Box B of FIG. 4) as closely as possibleconsistent with the desired quality of the etch. Best points for othersamples may differ, dependent on MO/TCO/TO thickness, mask adhesion,etc., but their determination will be within the skill of the ordinaryartisan in view of the teaching herein. In FIG. 4 Box A shows the rangeof the K/F preferred embodiment. Box B shows our determination of therange for fine line etching of large samples. The point 10 shows theetch bath composition of unpatented, unpublished, undisclosed 1960'stechnology from entities, no longer existing, preceding FeldmanTechnology Corporation (FTC). Point 12 represents FTC's later tradesecrete disclosure. The point X (H⁺=0.8 M; Fe⁻⁻⁻=0.4 M) shows our bestcomposition for a particular sample; it is included to illustrate thedesirability of optimizing in the direction of the lower right corner ofBox B, outside of Box A. Point 11 represents K/F's best reported result.

DESCRIPTION OF THE INVENTION

[0024] The following terms as used herein are defined as follows. SinceTO is preferred, TCO includes TO and may be referred to as TCO/TO. Also,since the invention is applicable to etching a metal oxide (MO) whichmay be stoichiometric, and/or opaque and/or non-conductive, the termMO/TCO/TO applies to the most general case. Since our work was primarilyconducted with TO, this term or TCO/TO is often used in the followingdescription of the invention.

[0025] SAMPLE: Refers to the sample being etched. It is typically aglass substrate covered with a MO, TCO, preferably TO film, typically upto 5000 Å thick, in turn covered by a mask which establishes a patternto be etched in the MO/TCO/TO. After the MO/TCO/TO not covered by themasking material has been etched, the mask is removed. Most of the workdescribed herein has been done with Shipley Microposit 1800 seriesphoto-resists (the product brochures and MSD sheets of which areincorporated herein by reference). A variety of other masks includingscreen-printed, solid film and other photo-resists may also be used. Apost-bake anneal at about 150° C. for 30 minutes enhances adhesion formost mask materials.

[0026] FINE LINE ETCH: In a Fine Line Etch, features in the patternbeing etched can be as small as can successfully be photo-printed.Experiments using the invention have provided features as small as 6micron lines and spaces. Diagramatically, a cross section of a fine linebefore etching is shown in FIG. 1 where m˜1-2 microns, t˜0.3 microns andl˜6 microns.

[0027] A condition for etching with Zn powder is that the Zn particle bein contact with the MO/TCO/TO; etching is observed to radiate fromcontact points. To picture the degree of contact note that commerciallyavailable Zn powder consists of irregularly shaped (˜4 micron)particles.

[0028] A PERFECT ETCH: A Perfect Etch with final cross section is shownin FIG. 2.

[0029] Perfect etch requires tight mask (resist) adhesion and noradiation of the etch under the mask. While tight mask adhesion can berealized, radiation of the etch under the mask can only be minimized.Optimally if etching is terminated when points A in FIG. 2 are reachedthe MO/TCO/TO under the mask will be undercut up to a distance t, givinga line profile as shown in FIG. 3.

[0030] THEORETICAL MINIMUM UNDERCUT: This is an etch where the undercutis ≦t as shown in FIG. 3.

[0031] PERFECT LINE DEFINITION: This is an etch that has uniformundercut over the entire pattern.

[0032] EXCESSIVE UNDERCUT: a situation occurring when etching under themask continues after unmasked areas have been etched down to thesubstrate.

[0033] BAD LINE DEFINITION: displayed when undercut is not uniform.

[0034] COMPLETE ETCH: An etch wherein unmasked TCO/TO has been etcheddown to the substrate over the entire sample.

[0035] INCOMPLETE ETCH: An etch wherein a thickness of unetched unmaskedTCO/TO remains over the entire sample.

[0036] PATCHWISE ETCH: An etch wherein the bulk of unmasked TCO/TO hasbeen etched down to the substrate, but islands of incompletely etchedTCO/TO remain.

[0037] Kato and Fukai (K/F) in Japan 4-69324. incorporated herein byreference, describe the original Zn-HCl procedure that their inventionimproves as follows: Zn powder is sprinkled onto a sample which is thenimmersed in a 10-20% (3.3-6.6 M H⁺) HCl solution. K/F state that etchingis initiated by the action of active (nascent) Hydrogen (H⁰)) producedfrom Zn+H⁺, which reduces Sn⁺⁴ in SnO₂ to metallic tin (Sn) at theexposed TO surface. They claim that the Sn dissolves in H⁺ to producemore H⁰, and thus the etch proceeds. In other words, the action of Zn+H⁺initiates the etch. the action of Sn +H⁻ continues it to completion.They observe that the etch suffers from excessive undercut (the termused herein for what they call “side etching”) and claim that the reasonlies in the production of H⁰ from dissolution of Sn under the mask. Theyargue that if the Sn under the mask is dissolved it can no longer be asource of excessive undercut. K/F's improvement is to add FeCl₃ to theetch bath to dissolve the Sn without H⁰ evolution. In the preferredembodiment of their invention the sample is sprinkled with Zn powder,then immersed for 30 seconds in a bath containing HCl in a concentrationrange of 4-18 wt. % (1.3-5.8 M H⁺) and FeCl₃ in the concentration rangeof 1-12 wt. % (0.04-1.03 M Fe⁺⁺⁻). Samples with a 1000 Å thick TO filmwith lines 30 microns in width and 20 mm long were etched with up to anorder of magnitude reduction in undercut compared with that obtainablewithout the FeCl₃. In their best reported etch they observed a 2000 Åundercut at line edges (twice the Theoretical Minimum described underour Definitions) using an etch bath with 2.8 M H⁺ and 0.8 M Fe⁺⁻⁻

[0038] In studying these phenomena we have determined that the chemistrydescribed by K/F, both in regard to the original etch and theirimprovement on it, is wrong. In the etch without FeCl₃ we observe that,on the time scale of the etch, the dissolution of Sn by H⁺ isinconsequential and therefore cannot be the source of H⁰ either for thedesired etch or the deleterious undercut. In fact, it is the H⁰ fromZn+H⁺ which is totally responsible for reducing TO to Sn, both in theoriginal etch and the K/F improved etch. The difference lies in thenature of the reduced Sn. In the original etch it is loosely attached tounderlying SnO₂ (it is easily rubbed off with a finger), has no sheen,is easily penetrable to H⁰, making no barrier to the etching whichradiates rapidly from Zn particle contact points with TO.

[0039] In the improved K/F etch the reduced Sn is quite differentphysically; it appears as a shiny film which cannot be rubbed offunderlying TO, inhibiting penetration by H⁰, slowing down the radiationof the etch. The improved K/F etch is more localized; a Sample, sparselycovered with Zn particles will be completely etched in the originalZn-HCl etch whereas it will be Patchwise Etched when FeCl₃ is added.More etch localization means less undercut. The fact that Fe⁺⁺⁺dissolves Sn is not the primary reason for the improvement of the etch.The primary role of Fe⁺⁺⁺ is to provide a source of Ferrous (Fe⁻⁺) ions,and the Fe⁺⁺ ions, not the Fe⁻⁺⁻ ions, are responsible for change in thephysical nature of reduced Sn.

[0040] Much of the concentration range in the preferred embodiment ofthe K/F invention cannot give a satisfactory Fine Line Etch. Especially,K/F's Samples are too small to demonstrate deficiencies of PatchwiseEtch which, in practice, pose severe problems with large Samples(surface areas to greater than 1 m²) of technological interest.Quantitatively, the Detailed Description of the Preferred Embodimentsbelow shows the concentration ranges of the invention etch that, wefind, can give a near optimum TO Fine Line Etch of large Samples (i.e.Samples having from 2 sq. in. to more than 1 sq. meter in surface area).Within the concentration ranges of the invention, the actual choices ofconcentrations, etc. should be tailored to the Sample, dependent on TOthickness, line width tolerances and mask adhesion, and is within theskill of the ordinary artisan in view of the teachings herein. Ourexperiments also show the desirability, for a given Sample, of finding aconcentration choice for Fe⁻⁺⁺ of 0.2-0.5 M and H⁻ of 0.5-2 M.

[0041] For MAX etching according to the invention (see infra)concentrations are preferably defined by Box B in FIG. 4. Morepreferable Fe⁺⁺⁺ values are≧0.27 such as 0.28, 0.29, 0.30, etc. and≦0.50such as 0.49, 0.48, 0.47, 0.46 and 0.45, etc. All values in Box B arespecifically incorporated herein by reference.

[0042] Particularly preferred concentrations of H⁻ (i.e., HCl) usefulherein for MAX and other etching methods are 0.5, 0.6, 0.7, 0.8, 0.9,1.0, 1.1, 1.2, less than 1.3, and 1.3-2.0 M. For MAX etching H⁺ ispreferable less than 1.4M. All ranges between all stated values, and allvalues between stated values, are included. Particularly preferred X(e.g., Fe⁺⁺⁺ (i.e., FeCl₃)) concentrations useful herein are 0.2, 0.25,0.3, 0.35, 0.4, 0.45, and 0.5 M including all values and ranges betweenstated values. For MAX etching X is preferably at least 0.27, morepreferably 0.3 and greater. 0.8 M H⁺ and 0.4 M Fe⁺⁺⁻ is particularlypreferred. 1.4 M HCl and 0.25 M FeCl₃ is also useful. Preferredconcentrations also include 0.3-0.5 M Fe⁺⁺⁺ used in conjunction with0.5-1.2 M H⁺. 1.45 M H⁺ with 0.26 M Fe⁺⁺⁺ is preferably excluded for MAXetching as is 1.4 M H⁻ with 0.52 M Fe⁻⁺⁺. MAX etching is useful for FineLine Etching at all above concentrations including 1.3-2.0 M H⁺ and0.2-0.5 M Fe⁺⁺⁺. None of the above concentrations need be excluded forMAP or MAPX (see infra) etching, and all are included. Concentrations ofP agent such as Fe⁻⁻ range from 0.01-1 M, including 0.1, 0.2, 0.3, 0.4,etc., M.

[0043] A key feature leading to the discovery of the present inventionwas the recognition that quality of the etch is governed by the physicalcharacteristics of the reduced Sn. dependent on a specific component inthe etch bath which we call the Penetration Control (P) agent. In theK/F etch Fe⁺⁻ ions, produced in a reaction of Fe⁻⁻⁻ with Zn, act as theP agent. A second feature was the recognition of the role played by asecond bath component, an oxidizing agent (X) that has the ability, inacid solution, to dissolve Sn; in the K/F etch, Fe⁺⁺⁺ ions are the Xagent.

[0044] We discuss below four etch categories of wet etching technologyfor metal oxide films, with comments regarding preferred embodiments.Our discussion utilizes TO as an example although the results areapplicable to other TCOs and metal oxide films.

[0045] 1. M-A ETCH: (A metal-acid etch with no P or X agent whereM=metal and A=acid). The original Zn-HCl etch described by K/F is themost attractive implementation, although, in establishing the broadpattern described here, other electropositive metals and other acids areincluded.

[0046] 2. M-A-X ETCH: (an M-A etch bath is augmented with an oxidizingagent X). The X agent, in acid solution, can dissolve Sn (or other metalof a TCO or metal oxide film). We note that if X can dissolve Sn itinevitably reacts with a similar or more electropositive M. An exampleis the K/F etch in which M=Zn A=HCl and X=FeCl₃, with the inevitablereaction: Zn +2Fe⁺⁺⁺→2Fe⁺⁺+Zn⁺⁺, producing the Fe⁺⁺ ion which happens tobe a powerful P (penetration control) agent. In a broader pattern, otherchoices for M, A, and X may be used. An essential feature of an M-A-Xetch is that the introduction of a P agent, if at all, occurs in situand is fortuitous. K/F, for example, did not recognize the existence ofP agents and introduced Fe⁺⁺⁺ ions solely on account of their ability todissolve Sn without producing H⁰, because they thought that a reactionof Sn with H⁻ was producing H⁰ responsible for undercut.

[0047] 3. M-A-P ETCH: (an M-A bath is augmented by a P agent without anX agent requirement). For example, M=Zn, A=HCl, and P=Fe⁻⁺ (either ofFeSO₄ and FeCl₂ are inexpensive sources). These are choices in oneembodiment of our invention. In an M-A-P etch we have explicit controlover the P agent, but must recognize that, for TO, when etching iscomplete there will be Sn trapped under the mask. This is easilydissolved in a second acid bath containing an X agent. Thus we separatethe use of P and X agents with independent control over the actions ofboth. This separation opens up a wide range of P agents that can beused, no longer dependent on an M-X reaction. Useful concentrations ofFe⁺⁺ in the etch bath are 0.1-1.5 M (e.g., FeCl₂, FeSO₄, etc.).

[0048] 4. M-A-P-X ETCH: (an M-A bath is augmented by controlledintroduction of both P and X agents). Here we recognize that theinevitable M-X reaction can produce more of, or a different, P. In thecase that: M=Zn, A=HCl, P=Fe⁻⁻ and X=Fe⁺⁺⁺, an M-A-P-X etch improves thecomparable M-A-X etch because it offers an extra level of control overfunctions of the P and X agents.

[0049] Our invention is a consequence of studies conducted in all fourcategories of etch described above. Our inventions in the M-A-P andM-A-P-X range have no precedent. We also provide an invention in theM-A-X range which is outside the K/F technology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] The present invention is best described by a detailed expositionof its various embodiments. For clarity, and because most of our workhas been with M=Zn, P=Fe⁺⁺ and X=Fe⁺⁺⁺, our discussion centers on thesespecific choices. The invention, however, is not limited to thesechoices. Central issues are the procedure for bringing M (Zn) in contactwith the TCO or metal oxide such as TO, and mechanisms of etchtermination. (Recall that etching only occurs if M (Zn) is in physicalcontact with TCO/TO-albeit point-wise contact in view of the shape ofthe Zn particles-and that etching radiates through the body of theTCO/TO film from contact points.) M (Zn) particles can be brought to thesample before, after or at the same time as the etch liquid, or at twoor more of these times. M particles can be applied to the Sample surfaceby painting, out of suspension or by spraying. Given an etch mechanismof radiation from contact points, the process can be somewhat forgivingof non-uniformity-how forgiving is primarily dependent on the P agent(Fe⁺⁺) concentration, which controls the degree to which etching islocalized around contact points. Uniformity of M on the surface ofTCO/TO is preferred. Higher P/Fe⁺⁺ concentrations means tighterlocalization demanding more uniformity in the spread. The degree of M(Zn) particle adhesion-to themselves, to TCO/TO, to the mask, and howetch liquid is introduced will affect M uniformity. A Sample may beimmersed into a bath or the etch liquid can be sprayed on. If excess Mis applied out of suspension agitation of the sample in the bathprovides effective uniform coverage. The best procedure depends upon theSample, the production line, etc.

[0051] At this point we look at the four etch methods described aboveseparately, paying particular attention to how etching terminates.

[0052] 1. M-A ETCHING: When M=Zn, A=HCl, the etch radiates rapidly andextensively from Zn contact points because in this etch reduced Sn iseasily penetrated by the H⁰ reducing agent, forming no barrier tocontinued etching. For this reason there is no satisfactory terminationto the etch. Because Zn is in excess etching will continue after theunmasked substrate is reached producing excessive undercut. Etchingstops only when Zn contact ceases-either the Zn is used up (consumed byacid) or the sample is removed from the etchant and washed off. A Zn-HCletch can be ruled out for fine-line etching.

[0053] 2. M-A-X ETCHING: Dominant mechanisms when M=Zn, A=HCl, andX=FeCl₃ follow. At the sample surface Zn reacts with both H⁺ and Fe⁺⁺⁻producing H⁰ and Fe⁺⁺ ions. This gives a surface concentration of Fe⁺⁺ions which act as an effective penetration control (P) agent. This isessential to the success of the etch: indeed. Fe⁻⁻⁺ concentrations mustbe chosen on the basis of the surface concentrations of Fe⁺⁺ that theyproduce. (We note that Zn also reacts with Fe⁺⁺, reducing it to metallicFe which coats the Zn particle-a reaction which may also be involved inproducing the shiny film of Sn on the TO that acts as a barrier tofurther reduction). The reaction of Zn with Fe⁺⁻⁺ is vigorous andstrongly exothermic-to the point that Zn particles can be lifted fromthe sample before the etching is complete. In fact, at high enough Fe⁻⁺⁻concentrations the Zn will lift off the sample before any etching takesplace at all. At somewhat lower Fe⁺⁺⁺ concentrations Zn lift-off canleave a Patchwise Etch. Optimized results require that the Fe⁺⁺⁺concentrations be low enough to avoid a Patchwise Etch but high enoughto control penetrability of Sn to H⁰ to minimize undercut. When X=Fe⁺⁺⁺these competing demands can be met successfully. As the etch proceeds,additional Fe⁺⁺⁺ ions dissolve reduced Sn, allowing further etching.Were it not for this reaction, etching would stop when H⁰ is unable topenetrate the built up layer of reduced Sn. The Sn dissolution reactionthus permits a complete etch provided that the pitfalls of premature Znlift-off and Patchwise Etch are avoided. The etch termination mechanismis of considerable interest. When TO is etched down to the unmaskedsubstrate, metallic Sn not yet dissolved by Fe⁻⁺⁺ flakes off, physicallyremoving Zn particles from the sample. Zn particles still remaining atthe sample are inhibited from causing serious undercut by a metallic Snlayer trapped under the mask. Thus, while excess Zn is still presentafter the desired etch is complete it is prevented from damaging theetch before being consumed by acid.

[0054] The importance of avoiding a Patchwise Etch must beemphasized-the situation where small incompletely etched islands ofunmasked TO are surrounded by large areas etched down to the substrate.The remedy, of course, is a second etch; recoat the sample with Zn andreintroduce the etch liquid. But if Zn particles do not make contactwith the small islands, and this is a real possibility, the islands willremain. Meanwhile, where the Zn contact is made with TO at the edge ofmask lines, undercut and bad line definition ensues. To minimize thissecond problem it is important to leave any Sn trapped under the mask atthe end of the first etch in place for the second etch.

[0055] What emerges is that a number of chemical reactions and physicaleffects must be kept in balance to complete a successful Fine-Line Etchof a large Sample with an M-A-X etch. Particular note is made ofreliance on the M-X reaction in acid solution to produce an effective Pagent. That good quality fine-line etches can be achieved with anappropriate range of H⁺ and Fe⁺⁺⁺ concentrations in an M-A-X etch isremarkable although that range is quite restricted, invalidating all buta small peripheral range in the K/F preferred embodiment. K/F's invalidrange falls prey to the conditions we have just discussed, namely noetching with very high concentrations because of Zn lift-off before theetch starts, Patchwise etching as concentrations are reduced and beforethey reach the valid range because of premature Zn lift-off ornonuniformity induced by too vigorous reaction, and excessive undercutwith very low Fe⁻⁺⁻ concentrations because reduced Sn is too easilypenetrated by H⁰. FIG. 4 shows the point representing best choice (asdetermined by undercut, line definition and completeness of the etch)for our particular≅3000 Å thick samples, supplied by AFG Industries Inc.as their product““Comfort E²””, the specifications of the which areincorporated herein by reference. In finding the best point we wereguided by the principle of approaching the lower right corner (lowestconcentrations of ingredients) of our designated range (Box B of FIG. 4)as closely as possible consistent with the desired quality of the etch.Best points for other samples may differ, dependent on MO/TCO/TOthickness, mask adhesion, etc., but their determination will be withinthe skill of the ordinary artisan in view of the teaching herein.

[0056] 3. M-A-P-X: Our M-A-X studies with Zn, HCl and FeCl₃ haveidentified a H⁺, Fe⁺⁺⁺ concentration range satisfactory for a goodquality Fine Line Etch, with Fe⁺⁺⁺ concentrations of from ˜0.5 M tolower values where line imperfections begin to appear, in order to avoidthe Patchwise Etching that can result from excessively vigorous reactionof Fe⁺⁺⁺ with Zn at the surface. Our invention of the M-A-P-X methodallows the etch to proceed with a low Fe⁻⁺⁺ ion concentration; no longerdo we depend on the M-X reaction to produce a necessary concentration ofFe⁺⁻, its primary role in the M-A-X- method. In the M-A-P-X etch thedesired Fe⁻⁻ is controlled by direct addition of a ferrous salt (FeCl₂or FeSO₄) to the etch liquid. With this control the primary role ofFe⁺⁺⁻ is dissolution of the reduced Sn; its effect on the Fe⁻⁻concentration is secondary. Since satisfactory Sn dissolution proceedsat lower Fe⁺⁺⁺ concentrations than is required in a M-A-X etch, theconcentration range that induces premature Zn lift-off is easilyavoided. 0.1-2.0 M A (e.g., H⁺ as HCl, etc.) is useful when a P agent isused.

[0057] One particularly effective implementation of an M-A-P-X etchwhich avoids premature Zn lift-off is described as follows:

[0058] A sample, pattern up, is covered to a depth of 2 mm. with anetchant solution consisting of 0.5 M H⁺, 0.4 M Fe⁻⁻ and 0.1 M Fe⁻⁺⁻.Enough Zn to completely exhaust the etchant solution is added and thebath containing Sample, etchant solution and Zn agitated forapproximately 60 seconds at which time all visual chemical activityceases. The sample surface shows partial coverage of undissolvedmetallic tin which can be dissolved in a later step.

[0059] In this implementation, the concentration of P(Fe⁺⁻) increaseswith time into the etch due to reaction of Fe⁺⁺⁺ with Zn; thuspenetrability to H⁰ decreases with time. The concentration of H⁺ isreduced with time due to the reaction with Zn forming H⁰. Fe⁺⁻⁺ isdissolving metallic Sn at a rate slower than it is being formed, itsconcentration decreasing with time as it reacts with Zn slowing the rateeven further. At the 60 second mark, H⁻ and Fe⁺⁺⁺ have been completelyexhausted; metallic Zn, metallic Sn and metallic Fe (from Zn+Fe⁺⁺)remain in the bath. The net effect of the balance of chemical reactions,and the change of that balance as the etch proceeds, is an etch withclose to theoretical minimum undercut and close to perfect linedefinition. Moreover, it is a prescription oriented to qualitycontrolled production since the etch starts with well definedconcentrations, has a minimal physical effect on the mask and ends witha discardable or recyclable benign neutral solution.

[0060] 4. M-A-P Etching: In the M-A-P method the etch liquid containsonly acid and a penetration control agent; absence of the X agent meansthat Sn is not being dissolved. If the Sn is easily penetrated by theH⁰, etching proceeds until all M is dissolved with resulting disastrousundercut in a Fine Line Etch, the fate of an M-A etch with Zn and HCl.

[0061] Since the P agent (Fe⁺⁻ for example) controls penetrability, anM-A-P etch has the potential for building up Sn to the point ofimpenetrability. If this state is reached after unmasked TO is etcheddown to the substrate, a complete one-stage etch is achieved withundercut limited to the Sn under the mask. If the state ofimpenetrability occurs before unmasked TO is etched to the substrate, anincomplete etch is achieved; with Fe⁺⁺ as the P agent all unmaskedregions are covered by a layer of shiny Sn attached to underlying TO. Insome applications this is a desired result representing an invention inits own right. In a preferred application it is an intermediate on theway to a complete etch. As an intermediate stage, when etching stops,the sample is removed from the etch bath, washed and put into a Sndissolving (X) agent. (Acidified FeCl₃ is eminently satisfactory, butjust one of many possibilities.) In an M-A-P etch we step throughintermediate stages until the etch is complete.

[0062] The big advantage of an M-A-P etch is the controlled terminationmechanism which avoids problems arising from Zn contact with the sampleafter TCO/TO is etched down to the substrate: in fact, procedures thatkeep the Zn contact after etching are advantageous because premature Znlift-off responsible for Patchwise Etching is avoided. This argues forintroducing Zn after, or at the same time as the sample is brought intocontact with the etch liquid, rather than a Zn first process.

[0063] The separation of Sn dissolution from the etching step opens upmany possibilities for etching liquids because competing chemicalreactions that can affect the etch are reduced to a minimum. There is nolonger the complication of the M-X reaction, most likely more vigorousthan the reaction of M with A which produces H⁰, since X will be a morepowerful oxidizing agent than H⁺. While the product of the M-X reactionmay fortuitously produce an effective P agent at suitable concentrationas in the Zn-Fe⁺⁺⁺ case, in general this will not be the case.

[0064] The M-A-P method may be at a disadvantage if thick TO films canonly be etched in several steps. Then, in a production environment, ifan M-A-X or an M-A-P-X etch can give the required quality, one of thesemethods may be preferred. At this point optimum choices arc dependent onthe production line technology. As in M-A-P-X etching, 0.1-2.0 M A(e.g., H⁺ as HCl) can be used when P is present.

EXAMPLES

[0065] Here we describe examples of M-A-X, M-A-X-P and M-A-P etches withclose to theoretical undercut and close to prefect line definition.

Materials

[0066] An example of an etch liquid is a solution containing HCl(source: commercial 36.5 wt. % Muriatric Acid), FeSO₄ (source: 98+%),FeSO₄.7H₂O crystals and FeCl₃ (source: commercial 42Be′). We describethe makeup of the solutions herein as M(H⁻)/M(Fe⁻⁻)/M(Fe⁺⁺⁺); i.e., M A/M P/ M X; e.g., a 0.8/0.3/0.2 solution with molar concentrations 0.8,0.3, and 0.2 of H⁺, Fe⁺⁺ and Fe⁺⁺⁻ respectively is made by mixing 80 ml.of 36.5% muriatic acid, 83.4 g of FeSO₄.7H₂O and 67 ml. of 42 Be′FeCl₃and adding water to make 1 liter. An example of M-A-X etch bath (no P)would be written R/0/T where R=M H⁻ and T=M X(e.g., Fe⁺⁺⁺).

[0067] An etch liquid, when it contains Fe⁺⁺⁺, can be used as anoxidizing bath; for example, a 1.0/0/0.5 etch liquid containing 1.0 MH⁺, 0.5 M Fe⁺⁺⁺ will rapidly dissolve metallic tin.

[0068] Our reducing agent is powdered metallic Zn (Superfine-7 from U.S.Zinc Co., particle size: 4.1 microns).

[0069] Our samples use AFG Industries Comfort E²≅3000 Å TO film maskedwith Shipley Microposit 1800 series photoresists in a variety ofpatterns formed on a 15×15 cm. surface, with features, both masked andunmasked, down to 6 microns. Masked samples are baked at 150° C. for 30minutes prior to etching. Photoresists are known in the art.

Example 1 M-A-X Etch (Painted Zinc)

[0070] A sample is covered by a thin layer of Zn particles by paintingfrom a slurry of Zn powder in water, then dried. Under a microscope thisshows as an approximately single layer of Zn particles not uniform on a50 micron scale, but to the eye no regions are uncovered. The sample islowered carefully. Zn up, into an 0.8/0/0.4 etch bath to a depth of ≅1cm. or greater. A short (fraction of a second) delay is followed byvigorous reaction for several seconds; metallic tin flakes are seen torise to the surface of the bath; activity at the sample surface ceasesand dissolution of Zn and Sn lifted from the etch surface proceeds untilall activity stops (in less than 2 minutes). The desired etch has beencompleted long before activity stops. The sample is lifted from thebatch, washed and dried. Finally the mask is removed by dissolution inacetone.

Example 2 M-A-P-X Etch (Painted Zinc)

[0071] Proceed as in Example #1 using a 0.8/0.3/0.2 etch bath. Timingand observations are similar to those of Example #1 and the quality ofthe etch is marginally improved. The most important difference is thatthis 0.8/0.3/0.2 etch bath will avoid patchwise etch for a wider rangeof samples than the 0.8/0./0.4 bath used in Example 1.

Example 3 M-A-P-X Etch (Zinc Slurry)

[0072] A 15×15 cm. sample is laid, pattern up, into a flat tray ofslightly larger dimensions and covered with 2 mm. of 0.5/0.4/0.1 etchliquid. Approximately 2 g. of powdered Zn is sprinkled onto the sample,through the liquid from an oversized salt shaker. The tray is agitatedhorizontally so that the Zn rolls this wave and that over the samplesurface giving, over time, uniform exposure of the surface to Zn.Metallic tin is seen forming on the surface, some staying, some peelingoff. When activity ceases (in less than 2 min.) the liquid is nearlycolorless (the characteristic red-brown color of Fe⁺⁺⁻ has disappeared)and some metal (metallic Zn, metallic Sn and metallic Fe) remains. Incomparison with Examples 1 and 2 where the etch liquid is still activeafter dissolving all metal, in Example 3 the etch liquid is exhaustedand metal remains, mostly as Zn particles coated with Fe, black inappearance as distinct from the gray uncoated particles, but also as Snstill attached to the surface especially at mask lines. The sample isremoved from the tray, washed free of all Zn particles, immersed in an1.0/0./0.5 oxidizing bath to dissolve Sn attached to the surface, washedagain and dried. This Example 3 etch gives highest quality results withno danger of patchwise etch.

Example 4 M-A-P Etch (Zn Slurry)

[0073] Proceed as in Example 3 but use a 0.15/0.5/0.0 etch bath. In thiscase activity in the tray stops after partial etching. Areas to beetched are covered by a metallic Sn film bonded to underlying TO not yetetched. After insertion and removal from an oxidizing bath the samplecan be recycled through an identical series of steps and the recyclingcontinued until etching is complete. Results are excellent; the exampleis given to demonstrate the possibility of controlling the etch to thepoint of producing potentially useful Sn layers on TO and to show how toseparate the reduction and oxidization processes of the etch which arecombined in M-A-X and M-A-P-X etches. This separation may provenecessary when using other than Fe⁺⁺ as a P agent, as we have alreadyshown with Cr⁺⁺ and Cr⁺⁺⁺ ions.

[0074] Other metals, alone or in combination, can replace Zn in thisinvention, Al and Mg are preferred candidates. H₂SO₄ is useful as areplacement acid. The acid (A) used herein is a strong acid that isessentially completely dissociated in water (e.g., HCl, H₂SO₄). Acidsthat are also oxidizing agents can also be used but must be evaluatedfor additional, possibly deleterious, reactions in the etching stage.Other P agents include transition metal ions; however, the action of theP agent is not presently understood at any fundamental level. We haveshown that Cr⁻⁺ and Cr⁺⁺⁺ ions can be used. For X agents when used in anM-A-P etch where dissolving Sn is carried out in a separate bath, anyoxidizing solution in an acid bath will work-dichromate, permanganate,etc.

[0075] The following embodiments A-I are within the skill of theordinary artisan in view of the teachings herein, and are preferredembodiments of the invention:

[0076] A. The M-A-X method of etching fine line samples of masked metaloxide films in which a metal (M) and an etch liquid composed of an acid(A) and a metal dissolution agent (X) are brought into contact with thesample; in which the metal oxide is reduced to its metallic form throughthe action of active Hydrogen (H⁰) produced in the reaction of M with Aand in which the additional reaction of M with X produces an agent thatcontrols the penetrability of the reduced metal oxide metal to H⁰ inorder to control undercut (etching under the mask).

[0077] B. The method of Embodiment A wherein the metal oxide is tinoxide, the metal, M, is zinc and the etch liquid is composed ofHydrochloric Acid (HCl) in the concentration range 0.5-1.5 M H⁺ andFerric Chloride FeCl₃ in the range 0.2-0.5 M Fe⁺⁺⁺. Optimumconcentrations can be tailored to the properties of the sample wherebythe concentration of Fe⁺⁺ at the surface, generated in the reaction ofZn with Fe⁻⁺⁺, reaches the highest value possible before patchwiseetching occurs.

[0078] C. The M-A-P-X method of etching samples such as fine-linesamples of masked metal oxide films in which a metal (M) and an etchliquid composed of an acid (A), a penetration control agent (P) and ametal dissolution agent (X) are brought into contact with the sample; inwhich the metal oxide is reduced to its metallic form through the actionof active Hydrogen (H⁰) produced in the reaction of M with A, in whichthe penetrability of the reduced metal oxide metal to H⁰ is controlledby the concentration of P and in which the primary function of X is todissolve the reduced metal oxide metal.

[0079] D. The method of Embodiment C wherein the metal oxide is tinoxide, the metal (M) is zinc and the etch liquid is made fromHydrochloric Acid (HCl) in the concentration range 0.4-1.5 M H⁺, FerrousChloride (FeCl₂) and/or Ferrous Sulfate (FeSO₄) in the concentrationrange 0.2-1.0 M Fe⁺⁺ and Ferric Chloride in the concentration range0.01-0.4 M Fe⁺⁻⁺. Optimization of the concentration ranges are tailoredto the properties of the sample whereby the Fe⁺⁺⁺ concentration is aslow as possible consistent with a one stage etch in a desired time.

[0080] E. The M-A-P method of etching samples such as fine-line samplesof masked metal oxides in which a metal (M) and an etch liquid composedof an acid (A) and a reduced metal oxide metal penetration controlagent, (P) are brought into contact with the sample; in which the metaloxide is reduced to its metallic form through the action of activeHydrogen (H⁰) produced in the reaction of M with A, and in which etchingstops when the reduced metal oxide metal becomes impenetrable to H⁰.

[0081] F. The method of Embodiment E wherein the metal oxide is tinoxide, the metal (M) is Zinc and the etch liquid is made fromHydrochloric Acid (HCl) in the concentration range 0.1-1.5 M H⁻ andFerrous Chloride (FeCl₂) or Ferrous Sulfate (FeSO₄) in the concentrationrange 0.3-1.0 M Fe⁺⁺. Optimum concentrations may be selected consistentwith a desired thickness of TO to be etched. In an incomplete etch alayer of reduced tin bonded to the unetched unmasked TO results.

[0082] G. The method of Embodiment F with the substitution of an etchliquid made from Hydrochloric Acid (HCl) in the concentration range0.1-1.0 M H⁺ and Chromous Chloride (CrCl₂) and/or Chromic Chloride(CrCl³) in concentration range 0.1-0.5 M Cr⁺⁺ or Cr⁺⁺⁺ (Because ofreactions with Zn and H⁺ a dynamic balance of actual Cr⁺⁺ and Cr⁺⁺⁺concentrations is established during the etch).

[0083] H. The method of Embodiments E and F in which the reduced metaloxide metal is dissolved from the sample in an acid bath containing anappropriate oxidizing agent.

[0084] I. The application of methods in Embodiments E-H to progressivelyetch films of metal oxide too thick to etch in a single stage.

[0085] Further preferred embodiments of the invention, all of which arewithin the skill of the ordinary artisan in view of our teachings, are:

[0086] J. A method of etching a masked metal oxide film in which a metal(M) and an etch liquid comprising an acid (A) and a metal dissolutionagent (X) are brought into contact with the metal oxide film, whereinthe concentration of acid in said etch liquid is 0.5-2.0 M and theconcentration of said metal dissolution agent is 0.3-0.45 M, where A isH⁺, X is Fe⁺⁺⁺ and M is Zn.

[0087] K. The method of Embodiment J, wherein the metal oxide is tinoxide, the metal, M, is zinc and the etch liquid comprises hydrochloricacid (HCl) in a concentration of 0.7-1.0 M and ferric chloride (FeCl₃)in a concentration of 0.3-0.4 M.

[0088] L. The method of embodiment J, wherein the concentration of acidin said etch liquid is 0.5-1.0 M and the concentration of said metaldissolution agent is 0.35-0.45 M.

[0089] M. A method of etching a masked metal oxide film in which a metal(M) and an etch liquid comprising an acid (A), a penetration controlagent (P) and a metal dissolution agent (X) are brought into contactwith the metal oxide film, wherein said etch liquid comprises 0.1-2.0MA, 0.1-1.5 M P, and 0.1-0.5 M X, where A is H⁻, P is Fe⁻⁻ X is Fe⁺⁺⁺,and M is Zn.

[0090] N. The method of Embodiment M, wherein the metal oxide is tinoxide, the metal (M) is zinc and the etch liquid comprises hydrochloricacid (HCl) in a concentration of 0.6-1.0 M H⁺, ferrous chloride (FeCl₂)and/or ferrous sulfate (FeSO₄) in a total concentration of 0.4-1.0 MFe⁺⁺ and ferric chloride in a concentration of 0.1-0.4 M Fe⁺⁻⁺.

[0091] O. A method of etching a masked metal oxide film in which a metal(M) and an etch liquid comprising an acid (A) and a penetration controlagent. (P) are brought into contact with the metal oxide film, whereinsaid etch liquid comprises 0.1-2 M A and 0.1-1.5 M P, where M is Zn, Ais H⁺ and P is Fe⁺⁺.

[0092] P. The method of Embodiment O, wherein the metal oxide is tinoxide, the metal (M) is zinc and the etch liquid comprises hydrochloricacid (HCl) in a concentration of 0.2-1.0 M H⁺ and ferrous chloride(FeCl₂) and/or ferrous sulfate (FeSO₄) in a total concentration range of0.3-1.0 M Fe⁺⁺.

[0093] Q. The method of Embodiment O, wherein said etch liquid compriseshydrochloric acid (HCl) in a concentration of 0.1-1.0 M H⁺ and chromouschloride(CrCl₂) and/or chromic chloride (CrCl₃) in total concentrationof 0.1-0.5 M Cr⁺⁻⁺.

[0094] R. The method of Embodiment O, further comprising dissolvingreduced metal oxide in an acid batch.

[0095] S. The method of Embodiment J, wherein said method is appliedsuccessively to a metal oxide film.

[0096] T. The method of Embodiment M, wherein said method is appliedsuccessively to a metal oxide film.

[0097] U. The method of Embodiment O, wherein said method is appliedsuccessively to a metal oxide film.

[0098] In the invention etch baths are preferably aqueous and metaloxide-coated samples are preferably at least one square inch in area,more preferably larger. Preferred undercuts approach the thickness t ofthe film being etched, and include 3 t, 2 t, <2 t, 1.5 t, 1.2 t, 1.1 t,1.05 t, 1.01 t. etc. In carrying out etching the metal can be contactedwith the MO film before, simultaneously with, and/or after the MO comesin contact with the etch liquid (emersion, spraving, etc.).

[0099] It is to be understood that various modifications of the appendedclaims will be apparent to skilled practitioners and it is intended tocover such modifications as fall within the spirit of the those claims.

What is claimed is:
 1. The M-A-X method of etching a masked metal oxidefilm on a substrate in which a metal (M) is deposited on said film andan etch liquid composed of an acid (A) and a metal dissolution agent (X)are brought into contact with the sample and metal such that the metaloxide is reduced to its metallic form through the action of activehydrogen (H⁰) produced in the reaction of M with A and in which theadditional reaction of M with X produces an agent that controls thepenetrability of the reduced metal oxide metal to H⁰ so as to controlundercut.
 2. The method of claim 1 , wherein the metal oxide is tinoxide, the metal M, is zinc and the etch liquid comprises hydrochloricacid (HCl) in a concentration of 0.5-2.0 M H⁺ and ferric chloride FeCl₃in the range 0.27-0.5 M Fe⁺⁺⁺.
 3. The M-A-P-X method of etching a maskedmetal oxide film on a substrate in which a metal (M) and an etch liquidcomposed of an acid (A), a penetration control agent (P) and a metaldissolution agent (X) are brought into contact with the sample whereinthe metal oxide is reduced to its metallic form through the action ofactive hydrogen (H⁰) produced in the reaction of M with A, thepenetrability of the reduced metal oxide metal to H⁰ is controlled bythe concentration of P, and X dissolves the reduced metal oxide metal.4. The method of claim 3 , wherein the metal oxide is tin oxide, themetal (M) is zinc and the etch liquid comprises hydrochloric acid (HCl)in a concentration of 0.4-1.5 M H⁺, ferrous chloride (FeCl₂) and/orferrous sulfate (FeSO₄) in a concentration of 0.2-1.0 M Fe⁺⁺ and ferricchloride in a concentration of 0.01-0.4 M Fe⁻⁺⁻.
 5. The M-A-P method ofetching a masked metal oxide film on a substrate in which a metal (M)and an etch liquid comprises an acid (A) and a reduced metal oxide metalpenetration control agent (P) are brought into contact with the samplesuch that the metal oxide is reduced to its metallic form through theaction of active hydrogen (H⁰) produced in the reaction of M with A, andin which etching stops when the reduced metal oxide metal becomesimpenetrable to H⁰.
 6. The method of claim 5 wherein the metal oxide istin oxide, the metal (M) is zinc and the etch liquid compriseshydrochloric acid (HCl) in a concentration of 0.1-1.5 M H⁺ and ferrouschloride (FeCl₂) and/or ferrous sulfate (FeSO₄) in a concentration of0.3-1.0 M Fe⁺⁺.
 7. The method of claim 5 , wherein the etch liquidcomprises hydrochloric acid (HCl) in a concentration of 0.1-1.0 M H⁺ andchromous chloride (CrCl₂) and/or chromic chloride (CrCl₃) in aconcentration of 0.1-0.5 M Cr⁺⁺ and/or Cr⁻⁻⁻.
 8. The method of claim 5 ,in-which the reduced metal oxide metal is dissolved from the sample inan acid bath containing an oxidizing agent.
 9. The method of claim 6 ,in which the reduced metal oxide is dissolved from the sample in an acidbath containing an oxidizing agent.
 10. The method of claim 1 , whereinsaid method is carried out successively to progressively etch a film ofmetal oxide.
 11. The method of claim 3 , wherein said method is carriedout successively to progressively etch a film of metal oxide.
 12. Themethod of claim 5 , wherein said method is carried out successively toprogressively etch a film of metal oxide.
 13. The method of claim 1 ,wherein the metal oxide is contacted by the metal simultaneous with orafter contact with the etch liquid.
 14. The method of claim 3 , whereinthe metal oxide is contacted by the metal simultaneous with or aftercontact with the etch liquid.
 15. The method of claim 5 , wherein themetal oxide is contacted by the metal simultaneous with or after contactwith the etch liquid.